Currently, a typical microphone assembly used in portable phones (e.g., mobile or cellular phones) converts acoustical signals to analog audio signals, which are transmitted from the microphone assembly along a signal line to an external A/D converter for digitization. As the analog audio signals travel from the microphone assembly to the A/D converter, however, they are undesirably susceptible to electromagnetic interference (EMI) caused by the presence of high frequency signals (normally around 1-2 GHz). To reduce the effects of EMI, the current practice in the mobile phone industry is to use external capacitors to de-couple the high frequency signal to “clean” the analog audio signals before digitization. After digitization, the resulting digital output signals are largely insensitive to EMI. Accordingly, it is desirable to convert the acoustical signals to digital output signals as soon as possible to prevent EMI from degrading signal integrity.
Further, different microphone assemblies currently used in portable phones have different sensitivity levels and output impedances. Thus, portable phones are typically designed with only one type of microphone assembly in mind, and the microphone pre-amplifier drive levels are set in accordance with the output characteristics of the particular microphone assembly. It is not practical, therefore, to substitute one microphone assembly for another because the gain of the microphone pre-amplifier would have to be adjusted to accommodate a different microphone assembly with a different output characteristic from that of the original microphone assembly. Thus, a hardware modification or an analog level adjustment of the microphone sensitivity is typically needed to switch one type of microphone for another.
U.S. Pat. No. 5,796,848 discloses a digital hearing aid with a microphone. In order to avoid EMI an A/D converter is positioned within the microphone casing whereby the A/D converter is shielded against EMI. The solution suggested in U.S. Pat. No. 5,796,848 reduces the influence of EMI, but due to a small opening in the casing, which is necessary so as to allow acoustic signals to be sensed by the microphone positioned inside the casing, EMI may still influence signal processing.
Electrical connections to the assembly such as power supply and input/output interfaces may also be sources of introducing EMI into the assembly. Even though digital input and output connections are very insensitive to EMI they may act as carriers/antennas so that EMI is introduced to the otherwise shielded microphone assembly. As described in U.S. Pat. No. 5,796,848 power supply lines are de-coupled against EMI by adding external capacitors to the powers supply line. However, the digital interface can not be de-coupled effectively against EMI applying such capacitors.
Therefore, there exists a need for a microphone assembly that shields analog audio signals against the effects of EMI without the use of de-coupling capacitors and that provides enhanced interchangeability. It is an object of the present invention to provide such microphone assembly.